I noticed this oddity while using a [phasor~] at low frequencies to ping a resonant filter: [hip~] oscillates quietly for cutoff frequencies below 20. You also get an interesting "zing" when you step its input between DC levels.
hip~ 1 oscillation.pd
No biggie, just something weird I noticed. I bet the math behind it is interesting.

strange oscillation in [hip~ 1]

@jameslo @sebharmonik.ar wrote about the maths here....... https://forum.pdpatchrepo.info/topic/11850/explanationforlopobjectinthispatchrequested/12
David. 
@jameslo @whaleav I've been looking at this & I think it has to do with floatingpoint roundoff stuff.
I think hypothetically the[hip~]
reaches some point of equilibrium in the phasor's cycle, when the increment of the phasor offsets the highpass zero characteristic (but the pole part is still acting, so the pole + phasor increment kind of cancel out the zero, at a given state of the filter). But when the new internal value is subtracted from the last internal value it probably creates a periodic effect that's due to roundoff error when subtracting the 2. It probably depends on the current state of the filter and the current[phasor~]
values coming in. The effect is pronounced at low frequencies because the numbers being subtracted are larger due to the pole location being closer to 1.
that's my guess